In an image forming apparatus such as an electro-photographic copying apparatus and a printer, in recent years, a belt type fixing device has been developed. The belt type fixing device can set a long fixing time, so that even in a color image composed of superimposed toner images, a satisfactory fixing property can be obtained. Some belt type fixing device stretches an endless fixing belt between a heating roller and a separation roller, heats the fixing belt by the heating roller, and inserts a sheet of paper through a nipper section between the fixing belt and a pressure roller to heat, pressurize, and fix a toner image. The fixing belt, to obtain high-speed and satisfactory fixing, is required to shorten the warming-up time and retain a predetermined fixing temperature. For the purpose, the separation roller for supporting the fixing belt together with the heating roller is required to use a highly heat insulating roller to prevent the fixing belt from cooling and furthermore is required to reduce the weight to realize compactness, lightweight, and energy conservation of the apparatus.
Conventionally, in Japanese Patent Application Publication 2004-109649, an apparatus having a plurality of heat pipes installed inside a central part of a core bar of a fixing roller is disclosed. However, the pipes are structured so as to be installed inside the core bar and uniformly diffuse heat to the core bar. Namely, the pipes do not form a heat insulation structure between the hollows of the pipes and the core bar. Further, in Japanese Patent Application Publication No. 8-292669, a separation roller composed of a hollow pipe, inside which air is sent, for making contact with the outside of a fixing belt, suddenly cooling a sheet of paper after heating and fixing, and separating the sheet of paper from the fixing belt is disclosed. However, in the aforementioned separation roller, the fixing belt is also cooled together with the sheet of paper and the temperature of the fixing belt cannot be retained.
Furthermore, as a heat insulating separation roller, there are a roller composed of heat resistant silicone rubber or heat resistant sponge and a ceramic roller covered with a PFA (perfluoroalkyl vinyl ether) tube available. However, the heat resistant silicone rubber and heat resistant sponge have comparatively high thermal conductivity, thus a higher heat insulating effect cannot be obtained, and the warming-up time cannot be shortened. Furthermore, the ceramics roller produces a heat insulation effect by an air layer contained in ceramics, contributes to shortening of the warming-up time, though it is difficult to ensure the manufacturing accuracy, requires a damage prevention measure, thereby is comparatively expensive.
Therefore, in the separation roller using the fixing belt, a fixing device for preventing the temperature of the fixing belt heated by the heating roller from being taken by the separation roller during rotation of the fixing belt, retaining the fixing temperature of the fixing belt, shortening the warming-up time, improving the fixing efficiency, and obtaining a fixed image of high image quality is desired.